Biological fluid contained in the interstitial space of living tissue such as in the brain, nervous tissue, and other organs, often must be sampled for research and diagnostic purposes. Where ample fluid is available for sampling, generally direct withdrawal is practiced. However, in many instances, ample fluid is not available and the direct withdrawal technique cannot be used so that sampling must be achieved by indirect methods.
In one such indirect sampling technique, a medium fluid is injected in the region at the same rate as the "washed off" fluid is withdrawn. However, in the indirect sampling procedure, as well as in the direct sampling procedure, significant depletion of the chemical substances in the region of sampling occurs, or the prior sampling techniques often yield a sample of fluid which requires extensive treatment prior to analysis in order to attain worthwhile measurements.
Small diameter probes have been used to perform dialysis in living tissue as an alternative technique in an attempt to solve many of the aforementioned problems. This alternative technique is now known as "microdialysis". In microdialysis, the medium in which dialysis is performed can be made deficient of only the biological compounds which need to be sampled and the sampled biological compounds should be of a molecular structure small enough to pass through the dialysis membrane.
Prior devices which have been proposed or used for performing a dialysis sampling procedure have significant limitations with respect to reproducibility of test results and ease of insertion of the corresponding probes. Typically, such small diameter dialysis probes are introduced through appropriate insertion guides in the living tissue to be examined, and consist of essentially an inlet tube for supplying a continuous fluid flow, an outlet tube for removing fluid biological substances, and a dialysis membrane tip. The dialysis membrane is formed of a semipermeable material which is flaccid and extremely delicate when wet and easily collapsible. Accordingly, such probes are difficult to put in place, and potential inconsistencies in fluid flow because of possible deformations in the membrane produces inconsistent and unreliable test results. So called "loop" type dialysis probes, "side-by-side" and "concentric" probe configurations have been proposed in microdialysis probes, all of which have the aforementioned problems.
Prior attempts to solve these problems have not met with success. In some instances, the probe is inserted into the living tissue in a dry condition to avoid a flaccid fiber membrane. However, in such situations it is not known whether the probe will perform satisfactorily when fluid is pumped into the membrane during dialysis. Inoperability of such a probe requires the removal and reinsertion of an operable probe.
In other instances, an inflexible solid support of the dialysis membrane is provided using, for example, the fluid conducting tubes as the support member with only a window of an active dialysis membrane. In other instances, a skeleton-like support member is used to support the dialysis membrane during insertion of the probe and is then removed prior to the sampling dialysis procedure. These attempted solutions using either an external or internal support for the dialysis membrane contain one or more of the following limitations:
1. Limit the application of the probes in procedures with straight insertions into the living tissue since insertions through complex shaped insertion guides is impossible;
2. Cause a significant obstruction in flow path;
3. Require removal prior to use in order to allow a direct contact of the membrane with the biological fluids.
It is therefore desirable to provide a microdialysis probe which overcomes all of the disadvantages of prior known devices and which provides reliable and consistent test results.